1. Field of the Invention
The present invention generally relates to a communication system and, more particularly, to an improved system for communicating compressed data from a spacecraft to Earth.
2. Description of the Prior Art
As is known by those familiar with the art of advanced space communication the information which is gathered in a spacecraft, generally referred to as data, is first coded to be transmitted to Earth, where it is received at one or more ground stations. The received coded data is first decoded and thereafter processed to retrieve the original data which is in the form of a stream of bits. The coding of the data in the spacecraft and the decoding of it after reception on the ground is generally referred to by the well known term "channel coding". As is appreciated, the basic motivation for channel coding has been to reduce the frequency of errors in the output information bit stream for a given signal to noise ratio, E.sub.b /N.sub.o, or conversely, to increase the transmission rate, R.sub.b, at which information can be transmitted with a given error probability. For each channel coding technique the average bit error probability is generally plotted as a function of the signal to noise ratio (in db). These plots are generally referred to as the performance curves.
In the last few years many articles have appeared in various publications in which various channel coding techniques are analyzed and their relative merits highlighted. The following are but a few of prior art references:
A. a. j. viterbi, "Convolutional Codes and their Performance in Communication Systems", IEEE Trans. Commun. Technol., Volume COM-19, and part II, October 1971, pp. 751-772. PA1 B. j. a. heller and I. M. Jacobs, "Viterbi Decoding for Satellite and Space Communication", IEEE Trans. Commun. Technol., Vol COM-19, part II, October 1971, pp. 835-848. PA1 C. j. p. odenwalder et al., "Hybrid Coding Systems Study", Final Report prepared by Linkabit Corporation for Ames Research Center NASA, September 1972. This report is available to the public as NASA Cr 114,486.
Reference A is an excellent tutorial on a decoder, now generally referred to as the Viterbi decoder for use with a convolutional coder and a modulator and transmitter in the spacecraft and a receiver and demodulator on the ground, hereinafter generally referred to as the Viterbi channel. Extensive performance characteristics of the Viterbi channel for different constraint lengths, represented by K, and different code rates, represented by 1/.nu., are analyzed and plotted in reference B. Most of the curves in reference B are plotted under assumed ideal operating conditions in which the carrier phase tracking loop signal-to-noise ratio, represented by .alpha., is assumed to be infinity. In FIG. 15 on page 845 of reference B the performance of the Viterbi channel for K=7 and .nu.=2, i.e., a code rate of 1/2 for various values of .alpha. is plotted in terms of bit error rate vs E.sub.b / N.sub.o (in db). As seen therefrom for any desired bit error rate the required system's E.sub.b /N.sub.o increases (transmission rate drops) as a.alpha. becomes smaller.
Reference C is related to a hybrid coding system which is analyzed. The hybrid system, as shown on page 10 of reference C, includes a Reed-Solomon (RS) encoder which encodes data gathered at a remote location, e.g., a spacecraft, into RS codewords, each consisting of code symbols and parity symbols. These codewords are first interleaved by means of a buffer prior to being encoded by a convolutional encoder in the spacecraft. The received coded data on the ground is first decoded by a Viterbi decoder whose output is loaded into a deinterleaving buffer to reconstruct the RS codewords, which are then decoded by a RS decoder. The output of the latter is the fully decoded data which is then processed. Since the convolutional encoder and the Viterbi decoder along with the modulation and demodulation system have been defined herein as a Viterbi channel, the system described in reference C can be defined as a concatenated RS-Viterbi channel, or system. Hereinafter it may also be referred to by the simpler term, "the concatenated system".
Although in reference C the advantages of the concatenated RS-Viterbi channel over other known channels are discussed, it should be stressed that in reference C the performance of the concatenated RS-Viterbi channel are analyzed only under assumed ideal conditions, i.e., .alpha. = .infin.. Performance under non-ideal conditions are neither discussed nor suggested. Also, none of the above mentioned references consider the channel from a system's point of view, including the type of data which is to be communicated.
As is appreciated by those familiar with the art of information communications, it is generally desirable to reduce the number of bits which represent any information, e.g., a picture of a planet, and which have to be transmitted without significantly sacrificing information content. This is desirable, since by reducing the number of bits, more information can be transmitted to Earth during any given period of time. This can be achieved if the original data, gathered in the spacecraft, can be compressed to reduce the number of bits needed to communicate the information before any coding is performed. As is appreciated various compression techniques may be employed. Then, after the data is decoded on the ground it can be decompressed, based on the particular compression technique employed in the spacecraft, to provide non-compressed data which is finally processed. It is appreciated however that when communicating compressed data a much lower average bit error rate is generally required as compared with non-compressed data since a single error in the compressed data stream is often propagated by the data decompressor into many errors in the reconstructed data.